Introduction

The State example is an easy to follow example that shows how the modifying of one fact can cause another rule to fire, it also demonstrates Conflict resolution with Salience values.

Instructions for Running the State Example

In order to build the Drools project from source, you must do the following:

• Install Maven, Java and CVS. There are good instructions for doing so in my OpenEJB article.
• Check out the Project from CVS
• Navigate to the top-level drools directory:

• Build the project (this might take a few minutes the first time):

• Change into the drools-examples directory and run the State example:

Understanding the State Example

Each State class has fields for its name and its current state. The two possible states for each objects are:

• NOTRUN
• FINISHED

In the example we have four objects: A, B, C and D. Initially all are set to state NOTRUN. A Bootstrap process fires setting A to state FINISHED which then causes B to change to state FINISHED. C and D are both dependent on B - causing a conflict which is resolved by setting salience values.

Instantiate four State objects and assert them into the Working Memory. Notice we set dynamic to true, this means we do not need to use drools.modifyObject( fact ) instead this is done automatically via the JavaBean PropertyChangeListener.

We need to bootstrap the system by checking for the A object with a state of NOTRUN. This will cause the AtoB rule to fire.

After the bootstrap process with A being set to state FINISHED, AtoB fires causing B to be set to state FINISHED.

With AtoB fired and B now at state FINISHED, we now have a conflict. Both rules BtoC and DtoC are activated and eligible to fire. Because we want BtoC to fire first we give it a higher salience value.

Finally BtoD fires.